Mercury removal

ABSTRACT

A mercury absorbent comprising a metal sulphide, a support material, a first binder and a second binder, wherein the first binder is a cement binder and the second binder is a high aspect ratio binder having an aspect ratio &gt;2. A mercury removal process comprises contacting a mercury containing feed stream with the absorbent.

This invention relates to mercury absorbents and to a process for theremoval of mercury from a gaseous or liquid stream using saidabsorbents.

GB-B-1533059 discloses the use of a pre-sulphided absorbent comprisingcopper sulphide for the absorption of mercury from a natural gas streamcontaining mercury. The pre-sulphided absorbent is prepared by forming aprecursor comprising basic copper carbonate and a refractory cementbinder, and then contacting the precursor with a gaseous streamcontaining a sulphur compound, e.g. hydrogen sulphide, so as to fullysulphide the copper compound. The pre-sulphided absorbent is then usedto remove mercury from a natural gas stream. It is also shown that anabsorbent comprising copper in the reduced, i.e. metallic, state is lesseffective at absorbing mercury than the pre-sulphided absorbent.

EP 0480603 describes a process for the removal of mercury from a streamwherein an absorbent comprising a metal sulphide is prepared in situ,preferably by the stream also containing a sulphur compound therebyconcomitantly preparing the mercury absorbent and absorbing the mercury,such that the formation of ineffective and undesired metal compoundse.g. sulphates is avoided.

Such materials, especially those described in GB-1533059, undergoconsiderable physiochemical changes upon treatment with sulphurcompounds to form the metal sulphide phases known to be effective formercury removal. The physiochemical changes result in reduced crushstrength and an increased susceptibility to attrition, particularly withhigh metal sulphide containing materials. It is therefore desirable toincrease the crush strength whilst maintaining attrition resistance. Wehave found that a combination of binders and support material in theabsorbent overcomes the problems associated with prior art materials.

Accordingly the invention provides a particulate mercury absorbentcomprising a metal sulphide, a support material, a first binder and asecond binder wherein said first binder is a cement binder and thesecond binder is a high aspect ratio binder having an aspect ratio >2.

The invention further provides a mercury removal process comprisingcontacting a mercury-containing feed stream with a particulate mercuryabsorbent comprising a metal sulphide, a support material, a firstbinder and a second binder wherein said first binder is a cement binderand the second binder is a high aspect ratio binder having an aspectratio >2.

The absorbent may be sulphided ex-situ or sulphided in-situ according toknown methods. Hence the invention includes a particulate mercuryabsorbent precursor comprising a metal compound capable of forming themetal sulphide, a support material, a first cement binder and a secondhigh aspect ratio binder, that may be treated with sulphur compounds toform the metal sulphide and a mercury removal process comprisingcontacting a mercury-containing feed stream with a particulate mercuryabsorbent comprising a metal sulphide capable of absorbing mercury,wherein the absorbent is formed by contacting in-situ a particulateprecursor comprising a metal compound capable of forming the metalsulphide, a support material, the first binder and the second binderwith a stream containing a sulphur compound.

Whereas the precursor may be sulphided using a stream which contains asuitable sulphur compound to form the absorbent and then held in-situ ina non-oxidising environment, it is preferred, where the absorbent is notpre-sulphided, that the sulphiding of the precursor and the absorptionof mercury occur together, i.e. they are concomitant, thereby avoidingthe need for a separate sulphiding process and the subsequent storagedifficulties. Hence in a preferred embodiment the feed stream and thefirst stream are the same and the present invention may beadvantageously used on streams that contain both mercury and sulphurcompounds.

The present invention may be used to treat both liquid and gaseous feedstreams containing mercury. In a preferred process of the invention thefluid is a hydrocarbon stream. The hydrocarbon stream may be a refineryhydrocarbon stream such as naphtha (e.g. containing hydrocarbons having5 or more carbon atoms and a final atmospheric pressure boiling point ofup to 204° C.), middle distillate or atmospheric gas oil (e.g. having anatmospheric pressure boiling point range of 177° C. to 343° C.), vacuumgas oil (e.g. atmospheric pressure boiling point range 343° C. to 566°C.), or residuum (atmospheric pressure boiling point above 566° C.), ora hydrocarbon stream produced from such a feedstock by e.g. catalyticreforming. Refinery hydrocarbon streams also include carrier streamssuch as “cycle oil” as used in FCC processes and hydrocarbons used insolvent extraction. The hydrocarbon stream may also be a crude oilstream, particularly when the crude oil is relatively light, or asynthetic crude stream as produced from tar oil or coal extraction forexample. The fluid may be a condensate such as natural gas liquid (NGL)or liquefied petroleum gas (LPG). Gaseous hydrocarbons may be treatedusing the process of the invention, e.g. natural gas or refinedparaffins or olefins, for example.

Non-hydrocarbon fluids which may be treated using the process of theinvention include solvents, such as liquid CO₂, used in extractiveprocesses for enhanced oil recovery or decaffeination of coffee, flavourand fragrance extraction, solvent extraction of coal etc. Fluids, suchas alcohols (including glycols) and ethers used in wash processes ordrying processes (e.g. triethylene glycol, monoethylene glycol,Rectisol™, Purisol™ and methanol) may be treated by the inventiveprocess. Natural oils and fats such as vegetable and fish oils may betreated by the process of the invention, optionally after furtherprocessing such as hydrogenation or transesterification e.g. to formbiodiesel.

Gaseous feed streams which are susceptible to being treated byabsorbents comprising metal sulphide precursors may also include thosewhich inherently contain both mercury and a sulphur compound e.g.certain natural gas streams, or a mercury containing gaseous stream towhich a sulphur compound has been added to effect mercury absorption.Suitable liquid streams include mercury containing LPG and naphthastreams. Other fluid streams that may be treated includemercury-containing nitrogen, argon, helium and carbon dioxide.

Preferably the absorption of mercury is conducted at a temperature below100° C. in that at such temperatures the overall capacity for mercuryabsorption is increased. Temperatures as low as 4° C. may be used togood effect in the present invention.

The mercury may be in the form of mercury vapour, organomercuric, ororganomercurous compounds. Typically the concentration of mercury in agaseous feed stream is from 0.01 to 1000 μg/Nm³, and more usuallybetween 10 to 200 μg/Nm³.

The absorbent may usefully be prepared by combining a metal compoundcapable of forming a metal sulphide with the support material and thefirst and second binders in the presence of a little water to form aparticulate precursor which is then dried and sulphided. As statedabove, the sulphiding step may be performed on the dried materialex-situ to provide the final absorbent, or may be performed in situ, inwhich case the particulate precuror is installed and undergoessulphidation in the vessel in which it is used to absorb mercurycompounds.

The sulphur compound used to sulphide the precursor may be one or moresulphur compounds such as hydrogen sulphide, carbonyl sulphide,mercaptans and polysulphides. Hydrogen sulphide is preferred.

Where concomitant sulphiding and mercury absorption occurs, the amountof sulphur compound that is present depends on the type of sulphurcompound and metal compound used. Usually, a concentration ratio, asdefined by the ratio of sulphur compound (expressed as hydrogensulphide) concentration (v/v) to mercury concentration (v/v), of atleast one, and preferably of at least 10 is used so that the precursoris sufficiently sulphided. Should the initial concentration of thesulphur compound in the feed stream be below the level necessary toestablish the desired ratio of sulphur compound to mercury compoundconcentration then it is preferred that the concentration of the sulphurcompound is increased by any suitable method.

The metal sulphide is desirably one with a high capacity for mercury.One or more metal sulphides may be present. The metal may be any whichprovides a metal compound which shows a suitable capacity for beingsulphided and hence for mercury absorption. Examples of suitable metalsare iron, nickel and copper, preferably copper and nickel and inparticular copper. Certain other metals, however, are generally unableto provide either compounds which can be suitably sulphided, e.g.aluminium, or sulphided compounds which can adequately absorb mercury.Nevertheless, a compound of such an other metal may be present as abinding or support agent, which improves the structural integrity of theabsorbent, and/or as a promoter which enhances the sulphiding of theprecursor and/or the absorption of mercury by the absorbent.

A preferred absorbent composition comprises copper and zinc. Copper andzinc compounds may be added separately to the support and binders toprepare the absorbent precursor. Alternatively, a single copper-zinccomposition may be used. A support material such as alumina may also bepresent in such a composition.

Upon treatment with sulphur compounds the metal compounds in theabsorbent precursor react to form metal sulphides. CuS is a particularlypreferred metal sulphide. Absorbents used in the present inventionpreferably comprise copper in an amount 1-40% wt, preferably 1-20% wt,more preferably 5-15% wt Cu (based upon the sulphided composition).

The metal compound suitable for use in an absorbent precursor is onethat may be readily sulphided and may include the oxide, carbonate,bicarbonate and/or basic carbonate. A particularly suitable absorbentprecursor comprises basic copper carbonate (i.e. a copperhydroxycarbonate).

Support materials are desirably oxide materials such as aluminas,titanias, zirconias, silicas and aluminosilicates. Hydrated oxides mayalso be used, for example alumina trihydrate. Preferred supports aretransition aluminas such as gamma, theta and delta alumina. The supportmay be present in an amount 50-90% wt, preferably 70-80% wt (based uponon the sulphided composition). By using the first and second binders wehave found that the amount of support material may be increased comparedto prior art materials without sacrificing rate of mercury absorption,strength or attrition resistance.

In the present invention, the metal sulphide or precursor is combinedwith a support, a first binder and a second binder. The first binder ispreferably a cement binder, in particular a calcium aluminate cement. Bythe term calcium aluminate cement we include such calcium aluminatecompounds as calcium monoaluminate (CaO.Al₂O₃), tricalcium aluminate(3CaO.Al₂O₃), pentacalcium trialuminate (5CaO.3Al₂O₃), tricalcium pentaaluminate (3CaO.5Al₂O₃), dodeca calcium hepta aluminate (12CaO.7Al₂O₃)and high alumina cements which may contain alumina in admixture with,dissolved in, or combined with such calcium aluminate compounds. Forexample, a well-known commercial cement has a composition correspondingto about 18% wt calcium oxide, 79% wt alumina and 3% wt water and otheroxides. Another suitable commercially available calcium aluminate cementhas a composition corresponding to about 40% wt calcium oxide, about 37%wt alumina, about 6% wt silica and about 20% other oxides. The secondbinder is preferably a high aspect ratio binder having an aspectratio >2. By the term high aspect ratio we mean that the ratio betweenthe maximum dimension and the minimum dimension of the particles is >2.The particles may thus be plate-like where the length and breadth are atleast twice the thickness. Alternatively, and preferably, the particlesare acicular, wherein the average length is at least twice, preferablyat least 2.5 times, the breadth, e.g. having a “rod” configurationwherein the cross sectional dimensions, i.e. breadth and thickness areapproximately equal, or a “lath” configuration, wherein the thickness issignificantly less than the breadth. A suitable high aspect ratio binderis an aluminosilicate clay, preferably an aluminium-magnesium silicateclay, commonly referred to as an Attapulgite clay. Without wishing to bebound by theory, we believe that the acicular nature of this bindercomprising elongate particles with an aspect ratio >2 contributes to theimproved physical properties of the absorbent materials according to thepresent invention. We have found surprisingly that the combination ofthese two types of binder in combination with the metal sulphide andsupport are capable of providing absorbent materials of high crushstrength and low attrition, as well as suitably high rate of mercuryabsorption. The amount of the first binder may be in the range 5 to 15%by weight based on the un-sulphided absorbent precursor. The amount ofthe second binder may be in the range 1 to 10%, preferably 2 to 6% byweight on the un-sulphided absorbent precursor. Preferably, the relativeamounts of the binders are 2:1 to 3:1 first to second binder.

The absorbent comprising the sulphided metal compound may be in anysuitable physical form, e.g. as a granule, extrudate, or tablet so thatthe mercury-containing stream may be contacted with a bed of solidabsorbent particles. Particularly effective absorbents are thoseprepared from precursors containing unsulphided metal compounds having acapacity to be highly sulphided. It is preferred that the amount ofunsulphided metal compound present is such that the precursor may besulphided to achieve a sulphur loading of at least 0.5% w/w, e.g. from1-10% wt sulphur, although higher loadings of sulphur may be provided.

The absorbent precursor may be in the form of tablets formed by mouldinga suitable powder composition, generally containing a material such asgraphite or magnesium stearate as a moulding aid, in suitably sizedmoulds, e.g. as in conventional tableting operation. Alternatively, theshaped units may be in the form of extruded pellets formed by forcing asuitable composition, containing the absorbent precursor material andoften a little water and/or a moulding aid as indicated above, through adie followed by cutting the material emerging from the die into shortlengths. For example extruded pellets may be made using a pellet mill ofthe type used for pelleting animal feedstuffs, wherein the mixture to bepelleted is charged to a rotating perforate cylinder through theperforations of which the mixture is forced by a bar or roller withinthe cylinder: the resulting extruded mixture is cut from the surface ofthe rotating cylinder by a doctor knife positioned to give extrudedpellets of the desired length. Alternatively, and preferably, theabsorbent or absorbent precursor may be in the form of agglomeratesformed by mixing the absorbent precursor material with a little water,insufficient to form a slurry, and then causing the composition toagglomerate into roughly spherical, but generally irregular, granules.

If desired the absorbent or absorbent precursors may be heated ortreated in another way to accelerate the setting of the cement binder.

The different shaping methods have an effect on the surface area,porosity and pore structure within the shaped articles and in turn thisoften has a significant effect on the absorption characteristics and onthe bulk density.

The absorbent preferably has an average particle size within the range1-10 mm.

The invention is further described by reference to the followingExamples.

In all cases, absorbent precursor particles were prepared using agranulation technique wherein the solid components were combined with alittle water and mixed to form granules in a Hobart mixer. A copper-zinccomposition comprising basic copper carbonate and containing 60% wtcopper, and 25% wt zinc (expressed as oxides) and about 15% wt aluminawas used. Alumina trihydrate (ATH) was used as the support materialunless otherwise stated. Binders were added to these two components,sometimes as a mix of two binders to produce the desired affect. Binder1 in each case was a calcium aluminate cement having a CaO content ofabout 40% wt. Binder 2 in each case was an Attapulgite clay. Thegranules were dried prior to sulphiding. Unless otherwise stated, dryingwas performed at 105° C. in air for 16 hours after a period of 2 hoursat ambient temperature (ca 20° C.). The size range of granules obtainedwas 2.80-4.75 mm.

To assess the performance of each of the materials screened, thephysical properties of the 2.80-4.75 mm material were measured beforethe sulphiding step. Mean Crush Strength (MCS), Tapped Bulk Density(TBD) and attrition resistance of the precursor were measured. The MCSand attrition of the materials were also tested after each of thematerials was sulphided using H₂S.

Mean Crush Strength (MCS): This analysis was carried out using a CT5crush strength tester fitted with a 50 Kg load cell. The granulesselected for testing were sized from 3.35-4.00 mm taken from the2.80-4.75 mm bulk sample, this would reduce any effect of the granulesize on strength. 30 granules were chosen at random from the 3.35-4.00mm size range and tested. The average of these results was taken to bethe resultant value. The standard deviation was also recorded as ameasure of the variability within the sample.

Tapped Bulk Density (TBD): This was carried using a 100 ml measuringcylinder into which was poured approximately 60 ml of the test materialthen the cylinder was manually tapped to achieve a constant volume. Atthis point the mass of the materials and the volume it held wererecorded.

Attrition Testing: Attrition testing was carried out using the ‘tubemethod’. In the tube method, a 50 ml of sample of the sulphidedabsorbent was weighed accurately into a 37 mm id tube of length 25 cm.Following a 45-minute tumbling period, the sample was reweighed afterthe removal of the <1.00 mm fraction by sieving. The % weight loss owingto attrition was calculated based on the weight <1.0 mm and the totalweight of material at the start of the test. The tumbling period of 45minutes subjected the sample to 3600 falls.

Sulphiding of Materials: The precursor materials were taken to a fullysulphided state using 1% H₂S in an inert carrier gas, (typically N₂ orCH₄). In all cases the flow rate of the gas was 42 litres hr⁻¹ atambient temperature and pressure.

EXAMPLE 1

Samples were prepared with the following compositions (all amounts areparts weight).

Example Cu/Zn composition ATH Binder 1 Binder 2 Comparative 1 100 0 7 0Comparative 2 25 75 14 0 Comparative 3 25 75 0 14 1(a) 25 75 7 7

Results: Before Sulphiding

Example MCS (Kg) TBD (Kg/m³) Attrition loss (% wt) Comparative 1 1.280.77 1.4 Comparative 2 2.81 1.06 4.6 Comparative 3 1.42 0.92 0.6 1(a)1.94 0.945 0.42

Results: After Sulphiding

TBD Attrition loss Example Sulphur (wt %) MCS (Kg) (Kg/m³) (% wt)Comparative 1 21.1 1.13 0.76 0.8 Comparative 2 2.9 2.66 1.06 4.8Comparative 3 4.8 1.03 0.88 0.2 1(a) 3.64 1.68 0.92 0.6

From the combination of properties Example 1(a) shows high MCS and TBDand low attrition pre and post sulphiding. The Comparative Example 2 hasa high crush strength but a very poor attrition resistance (i.e. ahighattrition loss).

EXAMPLE 2

In this example, further variations of the binder ratio were studiedtogether with the use of hot (55° C.) and cold (ca 15° C.) water duringthe granulation process.

Cu/Zn Example Treatment composition ATH Binder 1 Binder 2 2(a) Coldwater 25 75 10 4 2(b) Hot water 25 75 10 4 Comparative Hot water 25 7514 0

Results

MCS (Kg) MCS (Kg) Attrition (% wt) Example (Unsulphided) (Sulphided)(Sulphided) 2(a) 2.40 2.36 0.76 2(b) 2.37 2.52 2.13 Comparative 3.062.98 5.17

The results show that by using the combination of first and secondbinders that the attrition losses are reduced markedly over the singlebinder sample. The use of hot or cold water does not appear to have hada significant effect.

EXAMPLE 3

A further variation studied was to increase the copper level from the25% level in the above formulations to 50%.

Example Cu/Zn composition ATH Binder 1 Binder 2 3(a) 50 50 10 4

Result

MCS (Kg) MCS (Kg) Attrition (% wt) Example (Unsulphided) (Sulphided)(Sulphided) 3(a) 1.87 1.42 1.37

EXAMPLE 4 Mercury Pickup Tests

Those formulations selected for study were;

1) Example 2(a). 2) Example 3(a).

3) A comparative Example (Example 1 Comparative 3), containing onlyBinder 2.

The materials were prepared for testing by crushing a portion of the2.8-4.75 mm precursor granules to grain size 1.0-2.0 mm, then fullysulphiding in 1.0% of H₂S in nitrogen at 42 litres/hr. Following thesulphiding stage, the H₂S was purged from the reactor. The testing wascarried out using a 25 ml charge of the above prepared absorbents, atLHSV=7.0 hr-1, using n-hexane saturated with elemental mercury to ca.1.0 ppm w/v for a total run period of 750 hours unless otherwise stated.This would allow comparison with standard runs carried out previously.During the test period, the reactor effluent was occasionally sampledand analysed for mercury slippage.

At the end of the test period, the beds were dried using a gentle flowof nitrogen, and removed from the reactor as 9 discrete sub-beds using avacuum method. Each of the sub-beds was analysed for total mercury,using an ICP-atomic emission spectroscopic technique, to allow themercury profile to be determined. A further check was made, calculatingthe average mercury concentration inlet the reactor during the run. Fromthe analysis of the mercury level on each weighed sub-bed, the weight ofmercury was calculated on each, and the summation gave the total weightof mercury passed through the bed during the run. Knowing the totalnumber of hours the run had lasted and the hexane flow rate in ml/hr,then the average [Hg] inlet the bed in ppb of Hg (w/v) would be;

$\frac{{weight}\mspace{14mu} {of}\mspace{14mu} {Hg}\mspace{14mu} {on}\mspace{14mu} {{beds}(g)}}{{{hrs}\mspace{14mu} {on}\mspace{14mu} {line} \times {flow}\mspace{14mu} {rate}\mspace{11mu} \left( {{ml}\text{/}{hr}} \right)}\;} = {{ppb}\mspace{14mu} {Hg}}$

The results were as follows;

Total Mercury on Discharged Sub-beds (ppm w/w) Example 1 Comparative 3Example 2(a) Example 3(a) 1 Exit Nil Nil Nil 2 Nil Nil Nil 3 Nil Nil Nil4 Nil 10 Nil 5 17 92 Nil 6 76 810 35 7 1635 6250 440 8 17040 1900 100009 Inlet 47800 36200 45300 Calculated Inlet 1230 986 929 [Hg] ppb (w/v)

1. A particulate mercury absorbent comprising a sulphide of iron, copper or nickel, a support material, a first binder and a second binder wherein said first binder is a cement binder and the second binder is a high aspect ratio aluminosilicate clay binder having an aspect ratio >2.
 2. (canceled)
 3. An absorbent according to claim 1 wherein the support material is an alumina or hydrated alumina.
 4. An absorbent according to claim 1 wherein the first binder is a calcium aluminate cement binder.
 5. (canceled)
 6. An absorbent according to claim 1 wherein the second binder is an Attapulgite clay.
 7. An absorbent according to claim 1 wherein the relative amounts of the first and second binder are in the range 2:1 to 3:1 (first to second binder)
 8. A method for making the mercury absorbent according to claim 1 comprising the steps of (i) combining a compound of iron, copper or nickel capable of forming a metal sulphide with a support material, a first binder and a second binder in the presence of water to form a particulate absorbent precursor material, (ii) drying the absorbent precursor material, and (iii) sulphiding the precursor material to form the metal sulphide from the iron, copper or nickel compound wherein said first binder is a cement binder and the second binder is a high aspect ratio aluminosilicate clay binder having an aspect ratio >2.
 9. A method according to claim 8 wherein the iron, copper or nickel compound capable of forming the metal sulphide is combined with the support material, the first binder and the second binder in the presence of water in a granulator.
 10. A method according to claim 8 wherein the sulphiding step is performed by reacting a sulphur compound selected from the group consisting of hydrogen sulphide, carbonyl sulphide, mercaptans and polysulphides with the iron, copper or nickel compound capable of forming the metal sulphide in the dried absorbent precursor material.
 11. A mercury removal process comprising contacting a mercury-containing feed stream with an absorbent according to claim
 1. 12. A particulate mercury absorbent precursor comprising an iron, copper or nickel compound capable of forming a metal sulphide upon treatment with a sulphur compound, a support material, a first binder and a second binder, wherein said first binder is a cement binder and the second binder is a high aspect ratio aluminosilicate clay binder having an aspect ratio >2.
 13. (canceled)
 14. A mercury absorbent precursor according to claim 12 wherein the support material is an alumina or hydrated alumina.
 15. A mercury absorbent precursor according to claim 12 wherein the first binder is a calcium aluminate cement binder.
 16. (canceled)
 17. A mercury absorbent precursor according to claim 12 wherein the second binder is an Attapulgite clay.
 18. A mercury removal process comprises contacting a mercury-containing feed stream with an absorbent comprising a metal sulphide capable of absorbing mercury and wherein the absorbent is formed by contacting in-situ a precursor comprising an iron, copper or nickel compound capable of forming the metal sulphide, a support material, a first binder and a second binder with a first stream containing a sulphur compound, wherein said first binder is a cement binder and the second binder is a high aspect ratio aluminosilicate clay binder having an aspect ratio >2.
 19. A particulate mercury absorbent according to claim 1, wherein the sulphide of iron, copper or nickel is a sulphide of copper and the absorbent further comprises zinc. 